1. Field of Invention
The present invention relates to free-space optical switches, and particularly to a scalable, non-blocking, low insertion loss, re-configurable optical switches.
2. Related Art
An optical switch is a device for controlling the routing of light from one input port to any one of a number of output ports. A free-space optical switch is one in which the optical path is predominantly in an unguided medium. A non-blocking switch is one in which any input port can be connected with any unused output port, e.g., there are no particular paths between any input port and unused output port that are blocked because of other connections supported within the switch.
There are many ways to control the propagation of light. One way is to change the propagation direction of the light within the switch to direct the light through free-space to a desired output port, such as by rotating refractive (e.g., prisms) or reflective (e.g., mirrors) elements. Typically, the switch size is described in terms of the number of input and output ports. For example, a 2xc3x972 switch would allow each of two input ports to be connected any of two output ports. Typical switch sizes range from 2xc3x972 to 32xc3x9732. Future projections, however, show a commercial demand for non-blocking switches as large as 2048xc3x972048 or more.
One important performance metric of optical switches is their insertion loss, defined (in dB) as 10 log (Ii/Io), where Ii is the optical power applied to the input of the switch and Io, is the optical power transmitted through the switch (arriving at the desired output port). Achieving a low insertion loss is an important goal for any optical switch design. This requires, for example, careful control of the input position and propagation direction of the light as it is inserted into the free-space medium, accurate control of the propagation direction of the light within the switch, and efficient collection of the light into the output port. In general, the free-space beam must arrive at the desired output port at the proper position and propagation angle to be coupled efficiently into this port. The effect of even small translations or angular deviations from the nominal values can lead to a reduction in coupling, and hence an increase in insertion loss.
Typical insertion losses range from 7 dB for small switches (e.g., 16xc3x9716) to 20 dB for larger switches. Larger switches exhibit higher insertion loss due to both diffraction effects and pointing-error-induced translation errors of the propagating beam increase as the optical pathlength is increased. The ability of a particular switch""s design technology to enable the fabrication of larger format switches with appropriately small insertion loss is defined generally as the scalability of the switch.
A related performance metric is the uniformity of the insertion loss over all of the possible connections within the switch. In a large switch, the insertion loss may vary between 3 dB and 7 dB over the entire range of switch connections. It is desirable to have a lower and more uniform insertion loss between any input to output optical path.
Currently, large format optical switches are developed using microfabrication techniques to create large arrays of small, movable, opto-mechanical two and three-dimensional structures. In a typical two-dimensional approach, a two-dimensional array of mirrors is used to deflect the light from any input fiber to any output fiber. For an Nxc3x97M switch, the mirror array contains a total of N times M mirrors. Every mirror is used only for one particular switch connection, and it is moved out of the way for any other switch state. As such, the typical two-dimensional approach can be non-blocking, although many are not, such as disclosed in U.S. Pat. No. 6,072,923, entitled xe2x80x9cOptical Switching, Routing, and Time Delay Systems Using Switched Mirrorsxe2x80x9d, incorporated by reference in its entirety.
Even if this approach is non-blocking, there is the problem of limited scalability, due mostly to the requirement of using the same number of mirrors as switch states. For an Nxc3x97M switch, Nxc3x97M mirrors need to be arranged in a two-dimensional plane to provide the desired optical paths. Larger size mirrors increase the optical path length and the size of the switch, while smaller mirrors increase the insertion loss. Thus, a typical two-dimensional switch will have unacceptably larger insertion losses and larger state-dependent losses (larger non-uniformity), as the number of input and output ports is increased. U.S. Pat. No. 6,097,859, entitled xe2x80x9cMulti-Wavelength Cross-Connect Optical Switchxe2x80x9d, which is incorporated by reference in its entirety, discloses a two-dimensional switch that has limited scalability. Further, because the two dimensional switch has Nxc3x97M mirrors, issues with manufacturing yield and operating reliability also become important as the number of ports in the switch is increased.
Three-dimensional approaches allow smaller optical pathlengths and greater uniformity in optical pathlengths, due in part to the ability to distribute the number of Nxc3x97M optical paths within a three-dimensional volume, instead of a two-dimensional plane. The effect of this is a reduction in insertion loss and insertion loss non-uniformity over that provided by a similar size, two-dimensional switch. However, current three-dimensional approaches also have problems, such as with size, scalability, and complexity.
In one approach, described in PCT Int""l Publ. No. WO 00/52835, entitled xe2x80x9cOpto-Mechanical Valve and Valve Array for Fiber-Optic Communicationxe2x80x9d, incorporated by reference in its entirety, the three-dimensional switch still requires a large number of movable mirrors, with little description on reducing insertion loss. Other types of three-dimensional switches that reduce the number of mirrors necessarily increases the complexity, such as by requiring each mirror to have a large number of discrete pointing positions, e.g., corresponding to each input or output port. This requires precise alignment of the fibers and lenses with respect to each other and with respect to the micromirrors. Furthermore, positioning and control of the micromirrors can become complicated, thereby increasing the cost and complexity of designing and making such switches.
Other current types of optical switches utilize feedback to accurately adjust the propagation direction of the free-space beam, such as disclosed in U.S. Pat. No. 5,206,497, entitled xe2x80x9cFree-Space Optical Switching Apparatusxe2x80x9d, and complex beam pointing circuitry, such as disclosed in U.S. Pat. No. 6,005,998, entitled xe2x80x9cStrictly Non-Blocking Scalable Matrix Optical Switchxe2x80x9d, both of which are incorporated by reference in their entirety. Both these types of switches introduce added complexity and expense to the switch.
Current optical switches typically also have fixed fiber inputs and outputs, i.e., one set of fibers are used as light inputs and another set of fibers are used as light outputs. This fixed configuration of inputs and outputs limits the flexibility of the switch.
Accordingly, it is desirable to make and have a large free-space optical switch without the disadvantages discussed above with current optical switches.
In accordance with one aspect of the invention, a free-space non-blocking optical switch includes two planar substrates facing each other, with each substrate having patterns of beam steering elements (e.g., moveable mirrors, either singularly or in an array) and fiber interfaces with collimating elements (e.g., lenses). Each of a plurality of input and output fibers has an associated lens and beam steering element. The fibers are held in lithographically-defined through-holes in the substrate, and the associated lenses are located by lithographically-defined kinematic points in the substrate. The kinematic points set the lateral position of the lens with respect to the fibers. The use of lithography to determine the relative placement of the critical optical elements increases the precision and reduces the manufacturing costs. Other embodiments utilize fibers and lenses that are pre-aligned so that these assemblies can be mounted on a machined part rather than on a micromachined part with lithographic precision. This is due to relaxed lateral tolerances (e.g., from xcx9c1 micron to xcx9c50 microns). In addition, it is possible that the lenses (with or without fiber) may be actively aligned, in which case the lithographic precision is not needed.
The light from an input fiber exits the fiber with a relatively large divergence angle. It is focused by the lens into a collimated beam and directed, in one particular embodiment, after reflection from a fixed mirror, to the beam steering element associated with this fiber. The beam steering element is then controlled so as to direct the light to the beam steering element associated with the desired output fiber. The beam steering element associated with the desired output fiber is then controlled so as to direct the light to the output fiber lens and thus into the output fiber. The establishment of a path through the switch requires the control of two beam steering elements. The use of multiple mirrors, each with multiple positions, reduces the total number of mirrors required. Thus, an Nxc3x97M switch can be formed with N+M, instead of Nxc3x97M, mirrors.
According to an embodiment, a three-dimensional optical switch is constructed from beam steering elements placed in a substantially circular pattern about a central point, with the corresponding fiber and lens pairs also located in a substantially circular pattern of larger diameter about the same central point. The input terminated fiber waveguides, lenses, and beam steering elements may be located on a separate plane from the output terminated fiber waveguides, lenses, and beam steering elements.
Alternatively, according to another aspect of the invention, the waveguides, lenses, and beam steering elements are located on, but not necessarily limited to, the same plane through the use of an opposing stationary mirror or mirrors which xe2x80x9cfoldxe2x80x9d the light back through the waveguides. Each beam steering element is moveable to direct light to at least some other beam steering elements or lenses. The xe2x80x9cfoldingxe2x80x9d mirror allows the switch to be re-configurable in that a switch with N+M fibers and N+M beam steering elements can be re-configured, either through software or hardware, to form 1 by (N+Mxe2x88x921), 2 by (N+M-2), . . . , N by M, . . . , (N+M-2) by 2, and (N+Mxe2x88x921) by 1 size switches. Thus, the fibers are indistinguishable as inputs and outputs and can be used as both. This advantageously increases the flexibility of the switch by allowing a switch to be configured as many different size switches. Larger switches can be made using mirrors that are multi-stage, which require a lower number of controllable positions per mirror than single stage mirrors. For an 3-stage mirror with the first stage having k controllable positions, the second stage having m controllable positions, and the third stage having n controllable positions, a k*m*n port switch can be formed from mirrors having only k+m+n controllable positions.
The use of circular symmetry within a three-dimensional optical switch reduces the variation in path length relative to that which is inherent in similar size conventional two-dimensional switches. The use of circular symmetry and an appropriate choice of the radius of the circles on which both the fiber and lens pairs and the beam steering elements lie allows for the use of beam steering elements which have a substantially constant elevation angle.
According to one embodiment, the beam steering elements are moveable mirrors whose aiming direction is determined through contact with lithographically-defined kinematic points. The use of lithographically-defined kinematic points allows precision aiming of the mirrors without the use of feedback control and circuitry.
The present invention will be more fully understood when taken in light of the following detailed description taken together with the accompanying drawings.